The countercurrent heating apparatus which is the object of this invention concerns baking furnaces which are referred to as circulatory-firing open furnaces, in which an assembly of chambers, often placed in two parallel lines, are heated by gases raised to high temperature, which circulate in hollow partitions. The partitions which divide the chambers into compartments are themselves placed in a number of lines which are parallel to each other and also to the lines of chambers.
The partitions in the same line are connected together, at their ends, to permit the gases to flow sequentially therethrough. The products to be heated are filled into the compartments and heat is transmitted thereto by conduction through the walls of the partitions. These furnaces are used in particular for baking carbon products such as, for example, the carbon anodes of aluminum electrolysis cells. In each compartment, the carbon products are protected from oxidation by a thick layer of small-grain carbon material which covers them and fills all the free spaces. The compartments remain open in their upper part.
One of the serious difficulties which is encountered in operating such furnaces is that of reproducibly performing a cycle involving heating and cooling the products, to enable them to be baked under optimum conditions. The temperature cycle which should be imposed on the products to bake them is generally well known, but the difficulty arises in ensuring that the products which are placed in different zones of each compartment are actually subjected to a temperature cycle that is close to the optimum cycle.
When baking carbon anodes, it is important in particular that they be raised to a maximum temperature of at least around 1100.degree. C. to achieve the desired physical-chemical characteristics. Any overheating of the anodes in the hottest zones may cause the physical characteristics of the anodes to be modified, in comparison with the characteristics of anodes which are baked at lower temperatures. The result of this is heterogeneity of the characteristics of the batches of baked anodes produced, which, as is known, has highly disadvantageous consequences on the conditions of operation of the aluminum electrolysis cells in which the anodes are subsequently used. In addition, uncontrolled overheating causes accelerated deterioration in the refractory linings, and that results in rapid deformation of the heating partitions, so that the service life is thus very substantially reduced.
In general, the partitions are heated in the zone referred to as the baking zone, by fuel injection means placed in the upper part of the partitions.
A jet of fuel such as a heavy fuel oil or a fuel gas is thus introduced into a current of preheated combustion-supporting gas, the jet of fuel or gas being oriented in the direction of flow of the combustion-supporting gas.
In many cases, the partitions include baffles for causing the gases to flow in a zigzag configuration, in a downward direction and then in an upward direction, and so on.
Experience has shown that this method of heating the partitions causes overheating in the lower part thereof. As already stated, such overheating results, on the one hand, in heterogeneity in the physical characteristics of the anodes which are baked in the chambers and, on the other hand, accelerated deformation of the partitions in the most heavily loaded zone, which results in a very substantial reduction in the service life thereof.
A way has been sought to avoid such overheating of the partitions in the lower zone thereof and also to obtain improved distribution of deformation stresses in the partitions, to prolong their service life by a substantial amount.